A major shift in direction with regard to the studies of endothelial factor production has occurred over the last year. We have investigated long-term murine marrow cultures for the possible role of endothelial cells in modulating stem cell maintenance. Our information thus far has not implicated endothelial cells as being critical but has implicated a radioresistant alkaline phosphatase positive epithelioid cell along with a macrophage-like cell as being critical "stromal" determinants of hemopoietic stem cell maintenance. We have isolated a cell line from these marrow-adherent cells that produces colony-stimulating factor-1 and another activity capable of stimulating giant macrophage colonies in soft agar and of inducing new marrow cell lines that in turn make similar factors. The parent cell line is termed TC-1 and two subclones have been derived from this cell line, one of which supports hemopoiesis directly, the other of which does not. Additional studies have shown that irradiation of normal murine marrow of stroma results in the evolution of megakaryocyte, granulocyte, monocyte, and blast colony-forming activities and that addition of mature nonirradiated stromal cells to the irradiated stromal cells blocks this activity. These stromal layers also produce factors active on the interleukin 3 responsive cell line FDC-P1, but studies with the antibodies to interleukin 3 suggest that the activity is not interleukin 3. These investigations locate two critical cells for marrow stem cell regulation and imply the possibility of an autocrine type regulator system for marrow growth. Further studies will be designed to investigate the biologic nature of the activities produced by marrow stromal cells and the biochemical identity of these activities. (J)